The present invention relates to a torque converter, and has particular reference to a lock-up torque converter, a torque converter employing a direct drive clutch.
Motor vehicles installed with an automatic transmission including a torque converter give an easy drivability but are poor in fuel economy because there is a slip within the torque converter between a pump impeller thereof and a turbine runner thereof. Therefore, there have been proposed lock-up torque converters in which during a certain operation mode such as high speed engine operation where engine cyclic torque variation is small and negligible, the turbine runner will be coupled directly with the pump impeller to prevent the occurrence of slip in the torque converter thereby to improve fuel economy.
The lock-up torque converters require, for the purpose of operating the clutch, a passage providing communication between a clutch chamber and a lock-up control valve mounted within an oil pump cover radially extending from a stationary sleeve.
According to a conventional lock-up torque converter disclosed in U.S. Pat. No. 2,793,726, an output shaft is formed with an axial passage and with a radial passage extending from the axial passage to the bottom of a circumferential groove formed around the output shaft. A bushing is mounted within an annular space between the output shaft and a stationary sleeve with an axial end thereof abuting a shoulder of the output shaft. The bushing, fixed to the stationary sleeve, surrounds the circumferential groove and is formed with a cutout which mates with the circumferential groove and with a passage formed in a pump cover communicating with a lock-up control valve. The stationary sleeve is formed at an inner surface thereof with an axial groove or recess. This axial groove runs above that portion of the bushing which is not formed with the cutout. Communication between the control valve and a clutch chamber is established through the passage within the pump cover, the cutout of the bushing, the circumferential groove of the output shaft, the radial passage opening at the bottom of the circumferential groove, the axial passage of the output shaft and a second radial passage formed in the output shaft and opening to the clutch chamber. A pump driving sleeve surrounds a portion of the stationary sleeve shaft and forms therewith a torque converter oil feed passage communicating with the inside of a pump impeller. Oil is fed to the pump impeller through the feed passage and discharged from a turbine runner through the annular space and the axial groove formed in the stationary sleeve.
In this known passage structure, the circumferential groove formed in the output shaft and the bushing with the cutout are necessary for providing communication between the radial passage opening to the axial passage formed within the output shaft and the passage formed in the pump cover communicating with the control valve.
A problem with this known lock-up torque converter is derived from the provision of the output shaft with the circumferential groove. The problem resides in that, in designing an output shaft, the diameter of the output shaft must be sufficiently large enough to compensate for the structural weakness caused by the formation of a circumferential groove around the output shaft. Thus, it is necessary to use an output shaft large enough to compensate for the structural weakness caused by the circumferential groove.